The present invention relates to a method of producing the latch bearing in a latch needle for textile machines, and more particularly to such a latch needle which includes a shank having a longitudinally extending latch slot laterally delimited by needle shank cheeks, and a latch having a bearing member or portion at an end thereof inserted into the latch slot and positioned correctly therein with the use of an elongate hinge pin which is laterally inserted into laterally extending bores in at least one of the needle shank cheeks and into the bearing portion of the latch and is securely fixed in the bores in said needle shank cheeks.
The invention also relates to an apparatus for implementing this method. A prior apparatus of this kind (German Offenlegungsschriften Nos. 3,546,037 and 3,606,962) includes a clamping device for a latch needle into whose longitudinal latch slot the latch has been pre-assembled. A hole punching device o one side of the clamping device is equipped with a punch able to penetrate through the needle shank cheeks which laterally delimit the latch slot and through the bearing portion of the latch. A coaxial counter punch which is longitudinally displaceably mounted in bearing means on a side of the clamping device opposite the punch cooperates with the punch and is advanceable toward the clamping device.
The term "textile machines" is here to be understood to mean stitch forming machines, particularly knitting machines, but also specialized sewing machines and the like, in which yarn, wire and similar thread-shaped material is processed. The term "latch needle" includes all yarn etc. processing tools in which a latch or similar latch-like element is pivotally mounted in the latch slot of a shank.
In the latch needles primarily employed in practice at present, the bearing portion of the latch is generally configured so that the bearing hole disposed in the latch slot of the needle shank is mounted so as to pivot on two pivot or hinge pins pressed out of the material of the needle shank cheeks (see U.S. Pat. No. 3,934,109 and British Patent No. 836,297). The manufacturing process for this type of latch bearing is relatively simple because the configuration of the pivot pins and the installation of the latch on the pivot pins takes place practically in one process step which requires only simple and sturdy tools. Therefore, this method is quite economical, with high production rates being attainable. However, due to the unpredictable flow behavior of the material of the needle shank cheeks which is pressed into the bearing hole of the latch during the formation of the pivot pins, the pivot pins produced in this stamping process and shaped to extend from the needle shank cheeks are more or less irregularly shaped in the region of their outer peripheral faces so that the percentage of load-bearing area of the inner walls of the bearing hole for the latch on the pivot pins is relatively small. Particularly when used in fast running high performance machines, the small percentage of load-bearing area and high dynamic stresses result in a high specific load per surface area which, in turn, is the cause of premature wear phenomena.
It is known that a significantly more accurate and more wear resistant bearing for the latch can be realized in that the latch is mounted on a continuous hinge pin which has a smooth, cylindrical outer peripheral surface (see German Pat. No. 3,600,621 or the corresponding U.S. Pat. No 4,723,425). However, latch needles equipped with such a hinge pin latch bearing have not found wide acceptance in the past because manufacture and assembly of such smooth, continuous hinge pins is extremely difficult in an industrial setup due to the extremely small size of these pins. To give an idea of the order of magnitude involved, the bearing hole diameter of the latch of finer needle sizes lies at about 0.28 mm while the length of the hinge pin is about 0.35 mm.
In a prior art method for the production of latch bearings in knitting machine needles (see German Offenlegungsschriften DE-OS 3,546,037 and DE-OS 3,606,962), the needle shank is provided, before or after making the longitudinal slot, for example, with a transverse bore extending only through one shank cheek. A hinge pin whose length corresponds to approximately 2/3 of the needle shank thickness is pressed into the bore and then extends through the latch hole to the abutment at the opposite shank cheek which is not provided with a bore. A subsequently applied impression which surrounds the bore in the form of a ring or a corresponding annular weld serves to securely fix the hinge pin in the bore. In this case, the hinge pin is punched out by means of a punch through a die which is aligned with the bore in the needle shank and with the latch hole on the side of the needle shank. The hinge pin is punched out of a flat wire moved past the said die and is pressed by the punch into the hole in the shank cheek. Aside from the fact that the cutting edge of the die is worn out after a short period of operation and must be reground, and the cutting edge diameter is unduly enlarged by the clearance angle present at the die, the punched out hinge pins do not have continuously smooth cylindrical outer peripheral surfaces. Moreover, the method can only be used up to a ratio of the length to the diameter of the hinge pin that is 1 or less.